1. Field of the Invention
The present invention relates to an electronic part used for mobile communications apparatuses and the like, and a method of production thereof, and more particularly to an electronic part, such as an acoustic surface-wave device, a crystal device or a piezoelectric ceramic device, which requires an oscillation space near the surface of the functional device chip thereof, and a method of production thereof.
2. Prior Art
As the progress of mobile communications, electronic parts, a kind of key devices constituting apparatuses, are requested to be made more compact, lower in profile and higher in performance.
FIG. 20 shows the structure of a conventional electronic part having a functional portion 2 as an oscillation surface on the main surface 21 of a piezoelectric oscillation device. A functional device chip 1 is secured to a package 206 via an adhesive 205. Electric conduction between the chip 1 and the package 206 is established by electrode interconnection portions 6 comprising thin metallic wires made of gold, aluminum or the like. Furthermore, a metallic protection cap 207 is joined to the package 206 by seam welding or the like so as to ensure hermetic sealing.
However, in the case of an electronic part with such conventional thin metallic wires, the package is required to have lands for connecting the thin metallic wires to the package and is also required to have a joint portion for joining a cover, thereby causing limitations when making the electronic part more compact. In addition, as the operation frequency of an electronic part, in particular, such an electronic part as an RF acoustic surface-wave device operating at radio frequencies, is higher, the inductance of the thin metallic wires becomes noneligible.
As an approach to cope with the trend of making electronic parts more compact and operable at radio frequencies, instead of the conventional wire bonding mounting method, face-down mounting of acoustic surface-wave devices has been attempted by Onishi, et al as described in "A 1.5 GHz-Band SAW Filter Using Flip-Chip-Bonding Technique", Proceedings on 1993 Japan IEMT Symposium, pp109-112).
A conventional electronic part produced by a face-down mounting method and a method of production thereof are described below. FIG. 21 is a schematic sectional view showing the structure of a conventional electronic part wherein a surface oscillation portion, a functional portion 2, is disposed as the above-mentioned functional device on one main surface of an acoustic surface-wave device. The electronic part is provided with conductive projection portions on electrode pads 3 formed on the functional device chip 1. Furthermore, face-down mounting is carried out as follows: an electrode connection member 6 obtained by transferring and applying a conductive adhesive 6b onto the conductive projection portion 6a is made contact with electrode pads 83 on a package or a circuit substrate made of alumina, glass-ceramics or the like, and face-down mounting was carried out so as to establish external conduction. Moreover, a sealing resin 7 was used to reinforce the adhesion, because the adhesion strength between the chip 1 and the circuit substrate 8 is weak. Besides, a highly viscous insulating resin is used for the sealing resin 7 so as to obtain an oscillation space 20 around the functional portion 2. Additionally, a metallic protection cap 207 is connected to the circuit substrate 8 so as to obtain hermetic sealing at the space of the functional portion.
However, the conventional electronic part has the problem of requiring strict control of the viscosity and charge amount of the sealing resin in order to maintain the space near the functional portion of the functional device chip. To solve this problem, as disclosed in Japanese Laid-open Patent Application No. 8-307197, a method is proposed wherein the sealing resin can be charged easily by providing a protection circuit substrate, that is, a space retainer having a dent or cavity on the inner surface corresponding to the vicinity of the functional portion of the chip.
As shown in FIG. 22, in a conventional electronic part, a hermetically sealed space 20 is maintained in the functional portion 2 by directly bonding the space retainer 5 having a cavity portion onto the main surface 21 around the functional portion 2 of the functional device chip 1. The cavity portion inside the space retainer is formed by the corrosion method in a surface region facing the functional portion at a ceramic piece made of such crystal, other piezoelectric crystal or glass as the chip material of the acoustic surface-wave device so as to obtain the space retainer 5, and direct joint by chemical covalent bond between the space retainer and the main surface around the functional portion is used. Since the functional portion of the chip 1 is protected by the space retainer 5, the sealing resin 7 can be fully charged, thereby making the control of the sealing resin easier.
The sealing resin can be controlled easily by providing the space retainer to form the sealing space. However, alkaline washing is necessary in the process of direct joint between the functional device chip and the space retainer. Therefore, an aluminum electrode cannot be applied to the functional portion of the chip, and the process of forming the space retainer and directly joining the space retainer becomes complicated, resulting in higher cost. In addition, on the main surface of the chip, electrode wires are formed between the functional portion and the electrode pads, generating stepped portions. The fringe portion of the hard and rigid space retainer is directly joined onto the main surface of the chip, while crossing the stepped portions of the electrodes. Therefore, it is difficult to fully obtain hermetic sealing in the space of the functional portion between the main surface and the fringe portion of the space retainer. In this respect of obtaining hermetic sealing, the protection cap was necessary just as in the case of the conventional device.
Accordingly, an object of the present invention is to form a space forming structure having sufficient hermetic sealing at low cost. Another object of the present invention is to provide a reliable electronic part produced by face-down mounting at low cost in the protection structure of the functional portion, and allowing easy control of the sealing resin, and a method of production thereof.